1. Technical Field of the Invention
The present invention relates to an image forming apparatus such as printers.
2. Description of Related Art
In recent years, there has been a growing demand for printers to be more sophisticated by adding such various functions as double-side printing function, composite printing function, and color printing function. On the other hand, a compact and inexpensive apparatus is also required though it is provided with only a single function. However, for such an apparatus with a single function, there is not prepared any optional unit of the above-mentioned functions to be attached to the apparatus. The user who has been using a printer which is provided with a single function is, therefore, obliged to prepare a high-grade apparatus besides the apparatus of a single function when it becomes necessary for the user to utilize sophisticated functions.
In constructing a high-grade printer, it may advantageously be arranged to provide a versatile printer with various functions basing on the inexpensive printer with a single function, however, such a printer has not been proposed yet.
In U.S. Pat. No. 4,972,236, there is disclosed an image forming apparatus wherein a plurality of image forming units are combined. In the apparatus, a plurality of image forming units are vertically stacked, and a paper feeding section and paper discharging section of an upper and lower image forming units are connected by a paper conveying unit provided with changeover means capable of changing over paper conveying paths. According to the image forming apparatus, double-side printing function, composite printing function, color printing function and the like may be arranged by means of conveying sheets of copy paper in various modes with the paper conveying unit. A sorting function may also be provided by utilizing the paper conveying unit which is provided with a sorter section.
In the image forming apparatus, however, when a sheet is jammed in the paper feeding path of the image forming unit and paper conveying unit other than the uppermost image forming unit and the paper conveying unit or when various maintenance operations are conducted, it is necessary to clear the jam and conduct a maintenance operation by opening upper frame of the image forming unit or opening upper cover of the paper conveying unit after each unit stacked thereabove is removed which causes the operations more complicated.